The manufacture of many photonics devices are based on the ability to create permanent photorefractive changes in transparent materials. For example, the development of Bragg grating reflectors within planar or linear waveguides such as single mode optical fibres is well known and has been described in various United States patents. For example, one type of a Bragg filter, is incorporated or embedded in the core of an optical fiber by a method disclosed, in U.S. Pat. No. 4,807,850. As is discussed in this patent, permanent periodic gratings of this kind can be provided or impressed in the core of an optical fibre by exposing the core through the cladding to the interference pattern of two coherent beams of ultraviolet light that are directed against the optical fibre symmetrically to a plane normal to the fiber axis. This results in a situation where the material of the fiber core has permanent periodic variations in its refractive index impressed therein by the action of the interfering ultraviolet light beams thereon, with the individual grating elements (i.e. the periodically repetitive regions of the core exhibiting the same refractive index behavior) being oriented normal to the fiber axis so as to constitute the Bragg grating.
Other more popular methods of writing Bragg gratings in optical fibre are taught by Anderson in U.S. Pat. No. 5,327,515, and by Hill in U.S. Pat. No. 5,367,588. Both Anderson and Hill utilize a phase mask or optical phase grating. An interference pattern is generated by impinging a single light beam on the phase mask. The optical waveguide to be processed is exposed to the interference pattern, leading to the formation of a Bragg grating in the waveguide. In all of these prior art examples, an optical fibre having a Ge doped photosensitive core is irradiated with UV light of a predetermined intensity and for a predetermined duration sufficient to obtain a substantially permanent grating therein.
Although these prior art gratings provide a useful function, it would be advantageous to be able to write a grating in an un-doped light transmissive substrate or waveguide such as a typical telecommunications optical fibre, or on a slab waveguide device.
Aside from the drawback of having to provide specialty optical fibre by way of doping the core of an optical fibre so that the core becomes photosensitive to UV light, or additionally exposing such doped fibres to H.sub.2 or Deuterium gas at high temperatures for a substantial duration and under substantially high pressures so that its core becomes more photosensitive, optical fibre having a grating impressed therein, in the traditional manner has be joined to the telecommunications fibre to which it is to be coupled with. Of course, H.sub.2 loading and splicing fibre adds the cost and to the associated signal loss by virtue of having a coupling or splice joint between two optical fibres.
Refractive index changes written in standard UV-photosensitive optical materials such as Ge-silicate glasses are normally limited to a refractive index difference .DELTA.n&lt;10.sup.-3. Recently, research has been directed toward elucidating the mechanism for photorefractive index changes in glasses upon exposure to UV light, and progress has been made toward developing materials with enhanced photosensitivity, e.g, hydrogen loaded specially-doped silicate glasses for waveguiding applications, or photorefractive gels for bulk diffractive elements. However each of these materials suffer in one way or another from inferior optical or mechanical properties compared with normal optical glasses. Often a curing process is required following UV exposure, which can cause shrinkage and distortion of the optically written pattern. Photrefractive gels, in particular, are limited in their application due to the non-permanent nature of the index change, with decays on a timescale of a few years.
An alternative mechanism which employs high-intensity ultra-fast pulses for creating permanent photorefractive changes in glasses has recently been explored by several groups of researchers. Such disclosure can be found in a paper by K. M. Davis, et al. in Opt. Lett. 21, 1729 (1966) and in a paper by E. N. Glezer et al in Opt. Lett. 21, 2023, (1996). Glezer et al reported refractive index changes of .DELTA.n.about.0.1 written in fused silica using tightly focused pulses with peak intensities.about.10.sup.13 W/cm.sup.2. The physical process that gives rise to this refractive index change appears to be due to the creation of free electrons through multi-photon ionization of bound charges, followed by avalanche ionization and localized dielectric breakdown as these fee electrons are accelerated by the intense laser field. Phenomenologically, this leads to a localized melting and compaction of material, and a concurrent increase in the index of refraction. Owing to the extremely high intensities of light required to activate this photo-refractive mechanism, work performed in this field has used pulses that are tightly focused to near-diffraction limited spots. While this allows high-resolution spatial localization of the refractive index change to a volume on the order of 1-10 .mu.m.sup.3, it also requires that the laser focus be scanned point-by-point throughout three dimensions to build up a complete hologrammatic pattern in the material. This is a great disadvantage for writing diffractive structures that have extended dimensions, since mechanical precision of .lambda./100 must be sustained across length scales up to centimeters. Over time-scales of minutes, slight drifts in ambient temperature can lead to thermal expansions or contractions that often limit the accuracy of the fabrication process. Since raster scanning is an inherently slow procedure, this technique is not well-suited toward writing large diffractive structures.
Providing a hybrid technique of utilizing standard phase masking techniques in combination with using ultra short high power femto-second pulses is problematic, since close coupling a phase mask to create an interference pattern in a sample is not feasible; the mask will experience optical damage due to the high peak intensity of light required at the sample position.
Hence, in accordance with this invention, the mask must be located remotely and the diffracted light accurately imaged onto a small spot at the sample.
Since a phase mask introduces high angular dispersion in the diffracted beams, due to the broad spectral content of ultra-short pulses, simply redirecting each individual diffracted beam so that they overlap in the sample, unfortunately results in a greatly reduced peak intensity as the spectral content of the pulse is distributed over a relatively large area.
Thus, in accordance with a preferred embodiment of this invention, an imaging system is provided that overlaps replicas of the short pulse without significant spatial or spectral aberrations, and without any element experiencing peak intensities within two orders of magnitude of those at the sample.
It is an object of this invention to overcome many of the aforementioned limitations within the prior art systems of inducing a refractive index change in a light transmissive material.
It is an object of this invention to provide a system and method for writing gratings and patterns distinguishable by way of having a plurality of refractive index changes in un-doped optical glass.
It is yet a further object of the invention, to provide a system and method for inducing a refractive index change region of a piece of light transmissive material that is not doped to become highly photosensitive.